<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>ID定义CSS和引用.</title>
  <style>
    #box{
      width: 360px;
      height: 200px;
      color: seashell;
      background: red;
    }
  </style>
</head>
<body>
<div id="box">红色的盒子</div>
</body>
</html>